package four;

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        while (in.hasNextDouble()) {
            double a = in.nextDouble();

            double height = Math.sqrt(Math.pow(a,2) - Math.pow(a/2,2));
            double s = a * height / 2;

            System.out.printf("%.2f\n",s);

        }
    }
}
